kernel-hardening - Re: [PATCH v3 3/7] arm64: Introduce uaccess_{disable,enable} functionality based on TTBR0

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Message-ID: <20160914085159.GA13121@leverpostej>
Date: Wed, 14 Sep 2016 09:52:29 +0100
From: Mark Rutland <mark.rutland@....com>
To: Kees Cook <keescook@...omium.org>
Cc: Catalin Marinas <catalin.marinas@....com>,
	"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>,
	Will Deacon <will.deacon@....com>,
	James Morse <james.morse@....com>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	AKASHI Takahiro <takahiro.akashi@...aro.org>,
	"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v3 3/7] arm64: Introduce uaccess_{disable,enable}
 functionality based on TTBR0_EL1

On Tue, Sep 13, 2016 at 01:45:21PM -0700, Kees Cook wrote:
> On Tue, Sep 13, 2016 at 10:46 AM, Catalin Marinas
> > +static inline bool system_uses_ttbr0_pan(void)
> > +{
> > +       return IS_ENABLED(CONFIG_ARM64_SW_TTBR0_PAN) &&
> > +               !cpus_have_cap(ARM64_HAS_PAN);
> > +}
> > +

[...]

> >  #define __uaccess_enable(alt)                                          \
> >  do {                                                                   \
> > -       asm(ALTERNATIVE("nop", SET_PSTATE_PAN(0), alt,                  \
> > -                       CONFIG_ARM64_PAN));                             \
> > +       if (system_uses_ttbr0_pan())                                    \
> > +               uaccess_ttbr0_enable();                                 \
> > +       else                                                            \
> > +               asm(ALTERNATIVE("nop", SET_PSTATE_PAN(0), alt,          \
> > +                               CONFIG_ARM64_PAN));                     \
> >  } while (0)
> 
> Does this mean that with CONFIG_ARM64_SW_TTBR0_PAN, even with ARMv8.1,
> a cpu capability bitmask check is done each time we go through
> __uaccess_{en,dis}able?

Catalin reworked cpus_have_cap() to use static keys [1], and that's
queued in the arm64 for-next/core branch [2].

So this should expand to a single branch or nop that we patch when we
detect the presence/absence of PAN. There should be no bitmap check.

Thanks,
Mark.

[1] http://lists.infradead.org/pipermail/linux-arm-kernel/2016-September/454025.html
[2] https://git.kernel.org/cgit/linux/kernel/git/arm64/linux.git/log/?h=for-next/core

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